Traditionally, argon is produced from air in a cryogenic air separation plant. From the cryogenic plant, in addition to the recovery of oxygen and nitrogen-rich streams, a crude argon stream containing between about 94-97% argon, about 3-5% oxygen, and about 0.01-1% nitrogen is also recovered. The crude argon stream is further purified to produce a high purity argon stream. In the first step of this purification scheme, crude argon is mixed with hydrogen and passed through a catalytic hydrogenation unit to react oxygen to form water. This water is then removed, and the remaining gas stream is sent to a cryogenic distillation unit to remove nitrogen. The cost of this cryogenic distillation unit can be fairly high, especially for small sized plants; i.e., less than 12 tons per day (T/D) of argon production, and it can be a substantial fraction of the total argon costs.
Attempts have also been made in the past to purify crude argon without the use of a cryogenic distillation unit to remove nitrogen. In these processes, both nitrogen and oxygen are removed by an adsorption technique. An adsorption bed capable of preferentially adsorbing nitrogen from its admixture with argon and oxygen, by virtue of its general dynamic selectivity for nitrogen, is used to remove nitrogen. Typical examples of these adsorbents are suitable ion exchanged zeolites such as calcium X, lithium mordenite, etc. A second molecular sieve adsorbent bed, which has kinetic selectivity for oxygen, is used to remove oxygen from the mixture. Examples of these molecular sieve adsorbents are some of the carbon molecular sieves, 4A zeolite at a cryogenic temperature, etc.
U.S. Pat. No. 4,144,038 discloses a process whereby crude argon from a cryogenic air separation plant is first passed through a bed of molecular sieve carbon for selective adsorption of oxygen. The unadsorbed effluent, lean in oxygen is subsequently passed through a zeolite bed for selective adsorption of nitrogen. In accordance with this process, both the adsorbents are packed in the same column, and regeneration of the column is performed by simultaneous vacuum desorption of both absorbents in the column.
U.S. Pat. No. 4,477,265 assigned to Air Products and Chemicals, Inc. uses a sequence of nitrogen and oxygen removal for the purification of argon. In accordance with the process described in this patent, nitrogen is first removed from the crude argon followed by oxygen removal from the resultant stream. Nitrogen-selective adsorbents are placed in a separate column from oxygen-selective adsorbents. For a given throughput of the feed gas, a lesser number of oxygen-adsorbing columns than nitrogen-adsorbing columns are used, thereby providing enhanced flexibility for the overall operation.
European patent application No. 83307013.9 discloses a process for obtaining high concentration argon from air by means of pressure swing adsorption, characterized by passing air through zeolite molecular sieve-packed adsorption apparatus and a carbon molecular sieve-packed adsorption apparatus in that order. The air feed is subjected to pressure swing adsorption operations independently in the molecular sieve-packed adsorption columns, to produce concentrated argon and a high purity oxygen product simultaneously.
Other adsorption-based processes for the removal of trace quantities of impurities from argon gas are disclosed in Japanese patent application Nos. 57-174022; and 57-215464.